1. Field of the Invention
The present invention relates to a method for manufacturing a piston of a variable-capacity type compressor.
2. Description of the Related Art
There has heretofore been known a variable-capacity type compressor which is so constituted as to vary the discharge capacity by adjusting the pressure in the crank chamber in which a swash plate (inclusive of a wobble plate) is contained, and by varying the angle of inclination of the swash plate as taught in Japanese Unexamined Patent Publication (Kokai) No. 62-191673.
The compressor of the above prior art comprises an arm member that moves with a drive shaft, a swash plate mounted to the drive shaft so as to be able to change an inclined angle and rotate with the arm member, a piston accommodated in the cylinder bore so as to reciprocatingly move therein, and a piston rod of which an end forming a ball joint is engaged with a slide surface of the wobble plate via a shoe and of which the other end is coupled to the piston. Here, in coupling the piston and the piston rod together, a hole is perforated in the central boss portion of the piston to fit the piston rod in the piston, piston rod is press-fitted, and the piston rod is prevented from escaping by using a knock pin.
However, if the piston and the piston rod after they are machined are simply press-fittedly coupled together, the overall length of the coupled unit, i.e., the length from the top surface of the piston to the outer end (or the center of the sphere) of the ball joint of the piston rod, is the sum of the length from the bottom surface of the fitting hole of the piston to the top surface of the piston and the overall length of the piston rod. Therefore, the overall length of the coupled unit is determined including production tolerance of the piston and the piston rod. Besides, the overall length of the coupled unit is not only affected by the degree of tolerance (that is simply accumulated) but is also affected by very small bucklings (brought to the contacting surfaces of the two members) caused by the pressure of press-in coupling. Consequently, therefore, there arises a problem of deviation in the top clearance in the cylinder bores.
Moreover, in a piston having an outer cylindrical shell portion of a relatively small thickness, reinforcing ribs are formed between the outer cylindrical shell portion and the central boss portion to suppress the deformation of the outer cylindrical shell portion. Therefore, if there exist the reinforcing ribs, the central boss portion expands due to the interference fitted by the piston rod that is press-fit, and the effect spreads through the reinforcing ribs to the outer cylindrical shell portion, causing the outer peripheral surface of the piston to lose precision.